echo-types: add Nix flake devShell for use flake

The .envrc declared `use flake` but no flake.nix existed, so direnv
failed. Add a devShell pinned to nixpkgs nixos-24.11:

- rustc 1.82.0 + cabal-install 3.12.1.0 (exact .tool-versions match),
  agda 2.7.0.1, ghc 9.6.6, just 1.38.0
- Agda libraries: standard-library (nixpkgs, reproducible) +
  absolute-zero (local ~/dev/repos checkout; no usable upstream pkg),
  wired via a generated AGDA_DIR/libraries in the shellHook
- .gitignore: ignore the generated .agda/ dir

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

.gitignore

@@ -1,3 +1,4 @@
 *.agdai
 arghda-core/target/
 .claude/
+.agda/

flake.lock

@@ -0,0 +1,71 @@
+{
+  # SPDX-License-Identifier: PMPL-1.0-or-later
+  #
+  # Development environment for echo-types. Consumed by .envrc (`use flake`).
+  # Toolchain mirrors .tool-versions (cabal / rust / just) plus Agda, which
+  # the Justfile and echo-types.agda-lib require.
+  #
+  # nixpkgs pinned to nixos-24.11: rustc 1.82.0 + cabal-install 3.12.1.0
+  # (exact .tool-versions match), agda 2.7.0.1, ghc 9.6.6, just 1.38.0.
+  #
+  # Agda library resolution (echo-types.agda-lib: depend standard-library
+  # absolute-zero): standard-library comes from nixpkgs (reproducible);
+  # absolute-zero has no usable upstream package, so the local checkout at
+  # ~/dev/repos/absolute-zero is registered via a generated AGDA libraries
+  # file in the shellHook. Clone github.com/hyperpolymath/absolute-zero there
+  # if missing.
+  description = "echo-types dev shell (Agda + Haskell + Rust + just)";
+
+  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-24.11";
+
+  outputs = { self, nixpkgs }:
+    let
+      systems = [ "x86_64-linux" "aarch64-linux" "x86_64-darwin" "aarch64-darwin" ];
+      forAllSystems = nixpkgs.lib.genAttrs systems;
+    in {
+      devShells = forAllSystems (system:
+        let
+          pkgs = import nixpkgs { inherit system; };
+          stdlib = pkgs.agdaPackages.standard-library;
+        in {
+          default = pkgs.mkShell {
+            name = "echo-types";
+            packages = [
+              pkgs.agda
+              pkgs.just
+              pkgs.ghc
+              pkgs.cabal-install
+              pkgs.rustc
+              pkgs.cargo
+              pkgs.rustfmt
+              pkgs.clippy
+            ];
+            shellHook = ''
+              # Agda library file: nixpkgs standard-library (reproducible) +
+              # local absolute-zero checkout (no usable upstream package).
+              export AGDA_DIR="$PWD/.agda"
+              mkdir -p "$AGDA_DIR"
+              _stdlib_agdalib="$(echo ${stdlib}/*.agda-lib)"
+              _az="$HOME/dev/repos/absolute-zero/absolute-zero.agda-lib"
+              {
+                echo "$_stdlib_agdalib"
+                [ -f "$_az" ] && echo "$_az"
+              } > "$AGDA_DIR/libraries"
+
+              echo "echo-types devShell ready"
+              command -v agda  >/dev/null && echo "  agda : $(agda --version 2>/dev/null | head -n1)"
+              command -v just  >/dev/null && echo "  just : $(just --version 2>/dev/null)"
+              command -v rustc >/dev/null && echo "  rustc: $(rustc --version 2>/dev/null)"
+              command -v cabal >/dev/null && echo "  cabal: $(cabal --version 2>/dev/null | head -n1)"
+              if [ -f "$_az" ]; then
+                echo "  agda libs: standard-library + absolute-zero (local)"
+              else
+                echo "  WARNING: absolute-zero NOT found at $_az" >&2
+                echo "           clone github.com/hyperpolymath/absolute-zero into ~/dev/repos" >&2
+                echo "  agda libs: standard-library only (absolute-zero MISSING)"
+              fi
+            '';
+          };
+        });
+    };
+}